They are related to sea urchins and sand dollars.
These burrowing animals are seldom seen alive.Their
strange skeletons are sometimes seen though.

Where
seen?
Living heart urchins are rarely encountered as they usually remain
buried in the ground, some as deep as 20cm below the surface. But
their dead skeletons are often seen on Pulau Sekudu and sometimes
on our other shores. These skeletons (called the test) are fragile.
Often only pieces are seen.

What are heart urchins? Heart
urchins belong to Phylum Echinodermata
and Class Echinoidea which includes
sea urchins and sand
dollars. They got their name because they have a heart-shaped
body. They are also sometimes called Sea potatoes, as that is what
they resemble too.

Heart urchin features: Like other
echinoderms, heart urchins have have tube feet and spines. Although
they have a five-part symmetry, heart urchins are not strictly symmetrical
along five axes (and thus sometimes called irregular echinoids). Heart
urchins can be considered oval, burrowing sea
urchins. But unlike sea urchins, heart urchins have an obvious
'front' end and 'back' end.

Heart urchins have an internal skeleton (called the test) formed out
of large ossicles (pieces made of calcium carbonate) fused together
into plates in multiples of five. The test is a rigid and hollow without
any internal support. Heart urchins have quite obvious spines, just
like sea urchins. A heart urchin's mouth is on one end of the oval-shaped
body. Its anus is on the opposite end of the body. All heart urchins
lack the specialised Aristotle's lantern jaws that sea urchins have.

Breathing petals: The petal design
on the upperside of a heart urchin is called a petaloid. In heart
urchins, these are usually made up of only 4 instead of 5 'petals'.
The fifth petal is more reduced and usually found at the back end
of the animal. The petaloid is a series of tiny holes in the skeleton.
Tube feet emerge through these holes and the heart urchin breathes
through these feet!

Built to burrow: Their shape is an adaptation for
burrowing just under the surface. Some may have long spines, others
shorter ones. These spines are moveable and specialised to function
like spades to dig into the sand or to move around. Heart urchins
have a special band of tiny spines (called the fasciole). The fasciole
creates circulation of water within the burrow underground, and also
produces a sheet of mucus that helps to cement the burrow walls. Some
may also have very long tube feet that reach up to maintain an opening
to the surface.

Hearty food: Heart urchins process
the edible bits found in the sand as they burrow. Tube feet near the
mouth are specialised to pick up edible bits. The disc-shaped end
of the tube feet have tiny finger-like projections. These tube feet
are wiped against an internal structure called a raker. Some heart
urchins remain in their burrows and feed on particles that fall down
the burrow shaft. The particles are trapped by a mucous belt that
draws these into the mouth.

Heart urchin babies:Heart
urchins have separate genders and are usually either male or female.
They practice external fertilisation, releasing eggs and sperm simultaneously
into the water. Some may brood their young in special sunken parts
of their body. Heart urchin undergo metamorphosis and their larvae
look nothing like their adults. The form that first hatches from the
eggs are bilaterally symmetrical and free-swimming, drifting with
the plankton. At this stage, they have several long 'arms' which are
believed to funnel food particles into the central mouth. They eventually
settle down and develop into a a tiny heart urchin.

Heart urchins are preyed upon by Helmet
snails (Family Cassidae) which have a gruesome way of capturing
and eating the heart urchins.

Status
and threats:None
of our heart urchins are listed among the threatened animals of Singapore.
However, like other creatures of the intertidal zone, they are affected
by human activities such as reclamation and pollution. Trampling by
careless visitors also have an impact on local populations.

Order
Spatangoida recorded for Singaporefrom
Wee Y.C. and Peter K. L. Ng. 1994. A First
Look at Biodiversity in Singapore.*additions
from Lane, David J.W. and Didier Vandenspiegel. 2003. A
Guide to Sea Stars and Other Echinderms of Singapore.

Heart
urchins seen awaiting identificationHeart urchins can only be positively identified
from examination of their internal parts and are thus difficult
to distinguish in the field. On this website, they are grouped
by general colour and pattern for convenience of display.

The
Echinoid Directory by Dr. Andrew B. Smith on the London Natural
History Museum website: everything you could possibly want to
know about heart urchins with lots of large close-up images and
explanatory diagrams.

References

Lane, David
J.W. and Didier Vandenspiegel. 2003. A
Guide to Sea Stars and Other Echinoderms of Singapore.
Singapore Science Centre. 187pp.